1. Field of the Invention
This invention relates to integrated circuits, and particularly to a lithographic mask design for enhancing line end resolution.
2. Description of Background
There are ongoing efforts to reduce the dimensions of integrated circuit features. Unfortunately, the ability to decrease the size of features is limited by the process used to pattern those features. Photolithography is the process by which the features of integrated circuits are currently patterned and involves transferring an optical image from a mask (or reticle) comprising opaque regions and transmissive regions to a photodefinable material. The photodefinable material possesses the ability to undergo a change in solubility in response to being exposed to appropriate radiation such as ultraviolet light.
Therefore, in photolithography, radiation can be passed through the transmissive regions of the mask to expose underlying regions of the photodefinable material and thereby alter the solubility of those regions. The photodefinable material can then be developed through chemical removal of either its exposed or unexposed regions, depending on the type of photosensitive material (i.e., positive tone or negative tone) being used. In this manner, a pattern is formed in the photodefinable material. One example of a photodefinable material is a photoresist, which in patterned form can be used to protect underlying regions of a layer of an integrated circuit from being removed by an etch technique. Once it has served its purpose, the photoresist layer can be chemically removed also.
The overall resolution of a photolithography process refers to the minimum feature size that can be adequately printed or “resolved” within specifications. This overall resolution depends on parameters such as the properties of the photodefinable material and any subsequent etch processes and the resolution of the optical lithography system, i.e., the ability to form a resolvable image pattern on the semiconductor topography. Due to the limits of this resolution, the pattern formed in the photodefinable material might not exactly match the pattern in the mask used to define this pattern, particularly as the size of the features in the pattern get smaller. For example, the ends of lines patterned in an integrated circuit device, e.g., an SRAM device, can be distorted or shortened due to optical effects.